GUIDE
Specifying stainless tube without getting it wrong: the eight points to state
An incomplete enquiry is paid for in back-and-forth, in lead time, and sometimes in a non-conforming delivery. Here is what a supplier needs in order to quote correctly first time.
1. The grade: quote the EN number or the UNS
AISI alone is ambiguous. The most expensive case is 316L: AISI 316L and UNS S31603 cover two distinct EN grades, 1.4404 and 1.4435, whose molybdenum and nickel contents differ. A pharmaceutical specification requiring 1.4435 and receiving 1.4404 is non-conforming, even though the certificate reads "316L".
The same problem applies to the H grades: a 347 and a 347H are not interchangeable, the carbon range and grain size of the H grade being precisely what deliver creep strength. An 800, an 800H and an 800HT are three grades under one common name.
Write the EN number, or the UNS, or both.
2. The size: do not mix series
Three systems coexist and do not convert exactly.
The metric series gives outside diameter and wall thickness in millimetres: the most explicit.
The ASME inch series gives a DN or NPS, which is a nominal diameter and not the real one, paired with a SCH that codes the wall thickness. A DN 50 SCH 40S has an outside diameter of 60.3 mm — not 50.
The sanitary series — DIN 11850, ISO 2037, SMS — have their own correspondences. A DN 50 DIN 11850 and a DN 50 SMS do not connect.
The classic error is giving a DN without naming the series, or converting a nominal diameter into a real one. In case of doubt, give the outside diameter in millimetres: it is never ambiguous.
3. The product standard
EN 10216-5 for European seamless, EN 10217-7 for welded, ASTM A312 for American welded and seamless piping, ASTM A213 and A249 for heat exchanger tube, ASTM A270 and EN 10357 for sanitary, ASTM A790 and A789 for duplex.
The standard sets tolerances, testing, guaranteed properties and the content of the certificate. Without it, two quotations are not comparable.
4. The process: welded or seamless
If your calculation used a joint efficiency factor of 1, welded tube of the same wall thickness does not meet the same allowable stress. If the calculation has not been done, this is the moment to put it to the engineering department rather than decide by habit — specifying seamless without reason often means choosing a longer lead time for no gain.
5. The delivery condition
Annealed and pickled, solution annealed, polished, as-rolled. On sanitary grades, the internal finish and target roughness are specified at order stage: they determine the source, therefore the lead time, and cannot be recovered by treatment on site.
6. The length
Mill lengths, fixed lengths, or exact lengths. Fixed lengths generate chargeable offcuts; exact lengths extend the lead time. On a heat exchanger bundle the length is rarely a standard one and must be stated at enquiry stage.
7. The quantity
It does not just change the price, it changes the route: a few metres come from partner stock, several tonnes from a mill. Those are two different lead times, and a quotation that ignores this is an approximate one.
8. The certificate level
The standard is the EN 10204 3.1 material certificate. A 3.2, which requires an independent third party, is arranged with the source from enquiry stage. If your specification requires documented Melt & Pour traceability or a certificate of origin, say so in the tender: these are obtained upstream, never at delivery.
Checklist to copy into your enquiry
Grade (EN no. and/or UNS): .......................
Outside diameter × wall (mm): ....................
or DN / NPS / SCH + series: ....................
Product standard: ................................
Process: welded / seamless .......................
Delivery condition, finish, Ra: ..................
Length: mill / fixed / exact .....................
Quantity (m or kg): ..............................
Certificate: EN 10204 3.1 / 3.2 ..................
Melt & Pour traceability required: yes / no ......
Required date: ...................................The three most frequent errors
Designating by AISI alone. See point 1: 316L is the most expensive illustration.
Confusing nominal and outside diameter. A DN is not a dimension. Tube ordered on that basis does not connect.
Discovering the documentary requirement on receipt. The certificate is prepared with the order. A batch delivered without the required document cannot be accepted, and the document cannot be manufactured after the fact.
Frequently asked questions
- What if I don't know the grade already installed?
- Portable analysis gives the composition and identifies the family, and often the grade. Failing that, send us the context — medium, temperature, age of the installation, any marking on the tube — and we will help frame the enquiry.
- Can I enquire without knowing the wall thickness?
- Yes, by giving the service pressure and the medium: we will tell you which wall thicknesses are available. The final choice belongs to your engineering department, which owns the calculation.
- Is a 3.1 certificate always sufficient?
- For the large majority of industrial applications, yes. A 3.2 is required on certain pressure equipment and in certain project specifications. The specification decides.